Human cloning may never be possible, US scientists announced today. A team from Pittsburgh report in Science that - despite dramatic claims last year of the first cloned human babies - humans and their primate relatives may be just too difficult to replicate.

Since 1997, scientists have cloned sheep, mice, cattle, and even cats by transferring the nucleus of a parent cell into an unfertilised egg, and then kicking the process of cell division into action. When they tried it on female rhesus macaque monkeys, things went wrong. They started with 724 eggs from the macaque females, and made 33 embryos. They did not get a single pregnancy.

"When cells divide, there are basic things that are supposed to happen, and they just did not happen," said Gerald Schatten, of the University of Pittsburgh school of medicine. "Current techniques such as those used to create Dolly the sheep, mice and other domestic animals do not work in non-human primates. I don't want to say that it will never happen. Given enough time and materials, we may discover how to make it work. It just doesn't work now."

The research highlights difficulties that may lie ahead for researchers in so-called therapeutic cloning. Scientists in Britain have been given the green light to use embryos that are surplus to fertility treatments to understand more about how human beings turn - via embryo stem cells - from a single egg into trillions of cells of 300 different kinds in just nine months. Beyond that lies the hope that embryo stem cells could one day be used to treat neurodegenerative diseases and even make new heart tissue. But the news that reproductive cloning, which begins with essentially the same pathways, may be too difficult is likely to be greeted with relief.

"It is almost as if nature does not want us to clone ourselves," said Ronald Cole-Turner of the Pittsburgh theological seminary. "Nature seems to want cloning to be difficult, as if to suggest that sexual reproduction is intrinsically the better path."